Process of manufacturing a cold rolled steel sheet having excellent press formability

ABSTRACT

Based on the investigation as to the correlative relationship between the composition of a steel material, particularly the content of carbon and a soaking temperature for the hot rolling, the improvements of the stretch formability, deep-drawability and aging resistance of the cold rolled steel sheet and the peculiar behaviors of the effective additive ingredients under the above correlative relationship, a cold rolled steel sheet having excellent press-formability is obtained by soaking at 800°-1,100° C., a steel slab consisting of not more than 1.2% by weight of Si, 0.05-1.00% by weight of Mn, not more than 0.150% by weight of P, at least one of elements selected from the group consisting of Nb, Cr, Ti, Al, B and W in a total amount of 0.002-0.150% by weight in an extremely low range of not more than 0.005% by weight of C, followed by ordinary hot rolling, cold rolling and recrystallization annealing.

This application is a continuation of application Ser. No. 545,396,filed Oct. 13, 1983, and now abandoned.

TECHNICAL FIELD

This invention belongs to the technical field concerned with a processof manufacturing a cold rolled steel sheet having excellentpress-formability.

BACKGROUND ART

In general, the cold rolled steel sheets for press forming which areused for outer plates of automobiles, gasoline tank and the like arerequired to be excellent in stretch formability, deep-drawability andaging resistance.

The lower the yield strength (YS) and the higher the elongation (EL) andthe work hardening exponent (n value) as the material properties, themore excellent the stretch formability of the steel sheets.

The deep-drawability in the material properties is almost dominated bythe Lankford value (r value). The higher the r value, the higher thelimit of the deep-drawability.

On the other hand, it has been known that when there remain C,N in asolid solution state in the steel sheet, the trouble called "stretcherstrain" occurs upon working of press forming due to aging at roomtemperature. This necessitates the aging resistance, which is ordinarilyevaluated by using the aging index (AI). This index is represented bythe difference between the yield strength at 7.5% preliminary strain andthe yield strength after heat treatment of 100° C.×30 minutes. The steelsheets for use in press working are required to have the AI value of notmore than 3 kg/mm².

There have been heretofore proposed many processes of manufacturing thecold rolled steel sheets which are excellent in the above describedstretch formability, deep-drawability, and aging resistance. Forinstance, there is a method of box annealing a low-carbonaluminum-killed steel having a carbon content of about 0.04% by weight(the amounts of the steel ingredients being hereinafter referred tobriefly as "%"); and a method of box annealing or continuously annealinga steel sheet in which a carbonitride-forming element such as Ti, Nb orthe like is added into the extremely low-carbon steel having a carboncontent of not more than 0.1%.

However, these conventional processes have the common feature that thetemperature (hereinafter referred to as "soaking temperature") at whichsteel slabs are uniformly heated prior to the hot rolling is extremelyhigh near 1,200° C.

The reason why the soaking temperature is so high is as follows: In thecase of the low-carbon aluminum-killed steels, it is necessary tocompletely solid-solve AlN when soaking the steel slabs in order toobtain a high r value by the action of AlN precipitated upon boxannealing after the cold rolling. Meanwhile, in the case of theextremely low-carbon steel to which are added Ti or Nb, since the Ar₃transformation point at which the austenite phase is transformed intothe ferrite phase is extremely high near 900° C., the hot roll-finishingtemperature (FDT) must be high so as to avoid deterioration of thematerial properties due to the hot rolling at a temperature lower thanthe Ar₃ transformation temperature.

But, for heating the steel slab at a high temperature of about 1,200°C., a huge energy is not only required but also the higher soakingtemperature decreases the yield of the steel slab (due to the surfaceoxidation), and further promotes the interior oxidization in thevicinity of the surface of the steel slab, so that such a method has thedrawback that the trouble such as the surface defect, the surfacehardening and the like frequently occur.

As mentioned above, the heating of the steel slab at high temperatureleads to not only the consumption of much energy but also the surfacedefect and therefore there is strongly desired to establish the processof manufacturing the cold rolled steel sheets which lowers the soakingtemperature of the steel slab and gives the excellent press-formability.

There have been proposed several processes for manufacturing the coldrolled steel in which the soaking is carried out at a low temperature ofnot higher than 1,200° C., followed by hot rolling, for instance,Japanese Patent Laid Open Application No. Sho 49-129,622 (JapanesePatent Application No. Sho 48-43,856), Japanese Patent Laid OpenApplication No. Sho 51-59,008 (Japanese Patent Application No. Sho49-132,622) and Japanese Patent Laid Open Application No. Sho 55-58,333(Japanese Patent Application No. Sho 53-129,071). However, in any case,in order to make the hot roll-finishing temperature to be not lower thanthe Ar₃ transformation point, the soaking temperature must be actuallynot lower than 1,100° C. and in the very recent Japanese Patent LaidOpen Application No. Sho 57-13,123 (Japanese Patent Application No. Sho55-84,696), the soaking temperature of the steel slab is 1,100°-1,250°C.

To the contrary, in the low-temperature soaking in which the lower limitis 1,100° C., the above described effects for saving energy and avoidingthe decrease in the yield are suppressed to an extremely small degreeand the material properties of the cold rolled steel sheets are notsufficiently improved as described hereinafter.

In addition to the above, Japanese Patent Laid Open Application No. Sho53-64,616 (Japanese Patent Application No. Sho 51-140,532) discloses aprocess of manufacturing a steel sheet having an r value of 1.17-1.20 inwhich a rimmed steel slab having C of 0.05-0.11% is soaked at980°-1,050° C., and finished at a temperature of 710°-750° C. JapanesePatent Laid Open Application No. Sho 56-15,882 (Japanese PatentApplication No. Sho 55-60,713) discloses a process of manufacturing asteel sheet having an r value of 1.1 in which a steel slab having C of0.03% and Al of 0.05% is soaked at 950° C. and finished at a temperatureof 750° C. However, they both relate to the manufacture of the steelsheets having an r value being as low as not more than 1.2 andessentially differ from the deep-drawing steel sheet aimed at by theinvention.

DISCLOSURE OF INVENTION

An object of the invention is to provide a process of manufacturing acold rolled steel sheet having excellent press-formability whichovercomes the above described drawbacks in the prior art in theproduction of the cold rolled steel sheet for press working, and enablesthe treatment at a temperature of 800°-1,100° C., which is far lowerthan that of the prior art.

The principal constitution of the invention is as follows: That is, theinvention relates to a process of manufacturing a cold rolled steelsheet having an excellent press-formability in which a steel slabcomprising not more than 0.005% of C, not more than 1.20% of Si,0.05-1.00% of Mn, not more than 0.15% of P, a total amount of at leastone element selected from the group consisting of Nb, Cr, Ti, Al, B andW being 0.002-0.15%, and the balance being Fe and incidental impuritiesis subjected to a soaking treatment at a temperature range of800°-1,100° C., and hot rolled and succeedingly cold rolled and annealedfor recrystallization.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 (A), (B), (C), and (D) are correlation views showing theinfluences of varied soaking temperatures of steel slabs in fundamentalexperiments for accomplishing the present invention upon an aging index(AI), an r value, an elongation (El), and a yield strength (YS).

BEST MODE OF CARRYING OUT INVENTION

First, the fundamental experiments made by the inventors will beexplained.

Two kinds of steel slabs having the compositions as shown in Table 1were prepared by continuously casting molten iron obtained through abottom-blown converter and an RH degassing furnace.

                  TABLE 1                                                         ______________________________________                                        Steel                                                                         sample Chemical composition (weight %)                                        No.    C       Si     Mn   P    S    N     Al   Nb                            ______________________________________                                        1      0.0022  0.012  0.12 0.012                                                                              0.006                                                                              0.0025                                                                              0.032                                                                              0.008                         2      0.0061  0.010  0.13 0.010                                                                              0.005                                                                              0.0033                                                                              0.021                                                                              0.011                         ______________________________________                                    

After the above two kinds of the steel slabs were once left to be cooledto room temperature, and then soaked in a soaking pit.

The soaking temperature was varied over a range of 750°-1,250° C., andthe soaked steel slabs were hot rolled by means of a rougher consistingof 4 row rolls, and passed to a hot finisher consisting of 7 row rollsat two hot roll-finishing temperatures (FDT) of about 900° C. and about710° C., and coiled as steel strips having a thickness of 3.2 mm at aconstant temperature of about 500° C.

The hot rolled steel strips were pickled and cold rolled into coldrolled sheets having a thickness of 0.8 mm and then maintained at atemperature of 800° C. through continuous annealing and skin-pass rolledfinally at a reduction rate of 0.6% to obtain test samples.

The influences upon the material properties of the test samples due tothe differences in the soaking temperatures of the steel slabs are shownin FIGS. 1 (A), (B), (C), and (D). In the measurements of the materialproperties of the test samples, the tensile strength and the aging index(AI) were determined respectively using a tensile test piece of JIS Z22015 and a test piece taken in a rolling direction, and the r value,the elongation and the yield strength were expressed by the averagevalue of three directions, i.e., a rolling direction, and 45° and 90°directions to the rolling.

As seen from the measured results in FIG. 1, there is substantially nocorrelation in the test steel sample No. 2 having the carbon content of0.0061% shown in Table 1 between the soaking temperature within thetemperature range of 1,000°-1,250° C. and the material properties of thecold rolled-annealed sheet, and the r value of the low FDT steel is low.On the other hand, it has been found that the properties of the teststeel sample No. 1 having C of 0.0022% strongly depends upon the soakingtemperature of the steel slab. More specifically, when the results inthe case of a hot roll-finishing temperature (FDT) of 900° C.represented by the mark "o" are noticed, as the soaking temperaturelowers as from 1,250° C. to 1,100° C., and 1,000° C., the elongation andthe r value increased and the aging index (AI) and the yield strength(YS) lower and this indicates that the press-formability is conspicouslyimproved.

Meanwhile, when the measured results at the hot roll-finishingtemperature (FDT) being 710° C. expressed by the mark "•", are noticed,the material properties in the case of the soaking temperature beinghigher than 1,100° C., are fairly inferior to those in the case of thesoaking temperature being 900° C. However, when the soaking temperatureof the steel slab is not higher than 1,100° C., the material propertiesbecome very excellent as in those when the hot roll-finishingtemperature is 900° C. However, when the soaking temperature is as lowas lower than 800° C., it is apparent that the material properties arerapidly deteriorated.

This is an extremely important discovery. In the conventional process ofmanufacturing the cold rolled steel sheets for press forming, it hasbeen common knowledge that the hot roll-finishing should not be effectedat a temperature of lower than the Ar₃ transformation point at which thesteel is transformed from the γ-phase to α-phase, because such heattreatment causes the remarkable deterioration of the materialproperties. However, the Ar₃ transformation point of the test steel No.1 used in the above test by the inventors is about 830° C., andtherefore the above test results completely break the conventionalcommon knowledge.

The phenomenon observed in the test steel No. 1 in the experimentalresults shown in FIG. 1 is caused by setting the soaking temperature ofthe steel slab to a far lower range of 800°-1,100° C. than that of theconventional processes. For this reason, according to the invention, thesoaking temperature of the steel slab for the hot rolling is limited toa range of 800°-1,100° C. Based on the results of this fundamentalexperiment, the inventors have repeated the same experiment forconfirming the effect of soaking of the steel slab at the lowtemperature with respect to a variety of steel slabs having differentcompositions from the test steel No. 1 and confirmed that the effect ofthe low temperature soaking is more improved by limiting the steelcomponents as follows and that cold rolled steel sheets having excellentformability can be obtained.

C: not more than 0.005%

As seen from the properties of the test steel No. 2 having C of 0.0061%shown in FIG. 1, the effect in the low-temperature soaking disappears ifthe carbon content exceeds 0.005%. Thus, the carbon content is limitedto not more than 0.005%, preferably not more than 0.004%.

Si: not more than 1.20%

Si is an element effective for strengthening the steel. However, if itexceeds 1.2%, the hardness is conspicuously increased and the elongationlowers and the yield strength is raised. Thus, it is limited to not morethan 1.20%.

Mn: 0.05-1.00%

At least 0.05% of Mn is required to prevent the red shortness due to S,but if it exceeds 1.00%, it damages the ductility of the steel similarlyto Si. Thus, the content of Mn is limited to a range of 0.05-1.00%.

P: not more than 0.150%

P is high in the ability for strengthening the steel due to formation ofsolid solution and is an element having activity for increasing thestrength but if it exceeds 0.150%, it brings about conspicuousdeterioration of the spot weldability. Thus, the content of P is limitedto not more than 0.150%. Nb, Cr, Ti, Al, B and W: Total amount of atleast one of these elements being 0.002-0.150%.

These elements are important in the invention. The function and effectsof these elements are considered as follows:

(1) Any of these elements is a carbide, nitride or sulfide-formingelement and when the steel slab is soaked at 800°-1,100° C. according tothe invention, the forms of these precipitates extremely effectivelyinfluence the press-formability of the final product.

(2) Apart from the effect based on the formation of the aboveprecipitates, these elements behave similarly in view of the extremelygreat influence upon formation of micro-crystal grains and improvementof the texture when soaking the steel slab in the solid solution state.

These additive elements have been heretofore widely used for theimprovement of the properties of the iron steel materials, but it hasbeen considered that the addition effects varies depending upon theiraddition amounts and the combined addition with other elements, and alsodepend greatly upon the chemical compositions of the base steels.However, it has been found that these additive elements serve veryeffectively to improve the formability of the cold rolled steel sheetswhich have been subjected to the soaking treatment at a low temperatureof 800°-1,100° C. only in the case of an extremely low-carbon steelhaving a carbon content of not more than 0.005%, and that the functionaleffect is substantially equivalent in any elements. Therefore, theseelements may be added alone or in a combination of two or more elements.If the total addition amount is less than 0.002%, no effect is observed,while if it exceeds 0.150%, the effect is not increased in proportion tothe increased amount and the ductibity is adversely affected due to thehardening of the solid solution, so that the total addition amount islimited to a range of 0.002-0.150%. The optimum addition amount andcombination of these elements slightly differ depending upon theelements. Particularly, in the case of Nb or W, Al is within a range of0.005-0.08%, and in a combination of Nb and W, any of the total amountor the single element amount is preferred to be a range of 0.002-0.020%.When at least two elements of Cr, Ti, B and Al are selected, the totalamount thereof is optimum in the range of 0.002-0.090%.

The reason of the limitation on the elements in the steel according tothe invention has been explained but the balance consists of iron andincidental impurities besides the above elements.

Explanation will be made with respect to the steps for producing thecold rolled steel sheets having the above described composition whereinthe present invention is applied.

The steel making process is not particularly limited but the combinationof a converter and a degassing furnace is more effective in order tosuppress the content of carbon to not more than 0.005%.

The process of manufacturing the steel slab may be the conventionalslabbing, that is ingot making-blooming method or a continuous castingmethod.

With respect to the heating of the steel slab, it is important to soakit at a temperature range of 800°-1,100° C. If the soaking can becarried out within this temperature range, the method and apparatus forheating the slab are not limited and the temperature of the steel slabprior to the soaking is arbitrary. Accordingly, the steel slab may beone completely cooled to room temperature or one having a temperaturehigher than room temperature and it is merely necessary to soak the slabat a temperature range of 800°-1,100° C. by reheating. The soaking timeis not particularly limited and if the entire steel slab is heated tothe soaking temperature of 800°-1,100° C., the object can be attainedbut the soaking time is preferred to be from 10 minutes to one hour.

Therefore, with respect to the steel slab manufactured by the continuouscasting, when the temperature of the steel slab is not lower than 800°C., it is unnecessary to once cool and reheat, but it is merelynecessary to keep the temperature at a temperature range of 800°-1,100°C. or to gradually cool the slab to this temperature range. Therefore,no particularly heating furnace is necessary in the case of the steelslab obtained by the continuous casting, and it is possible to attainthe satisfactory effects only by regulating the cooling speed.

In the hot rolling of the thus soaked steel slab, no adverse effecttakes place on the material properties of the final cold rolled steelsheet so long as the rolling conditions such as rolling speed, rollingreduction, distribution of reduction in rolling, roll-finishingtemperature and coiling temperature and the like are within the usualranges.

However, if the finishing temperature in the hot rolling is too low, thedeformation resistance becomes high to make the rolling difficult, sothat it is preferable to be higher than 550° C. Further, since thesurface oxidized layer of the hot rolled steel strip formed untilcoiling after the finish rolling highly influences the surface profileof the final cold rolled steel sheet, the finishing temperature ispreferred to be as low as possible. Therefor, the finishing temperatureis preferably 550°-850° C. Since the steel containing an element orelements other than Nb and W is very low in the deformation resistancein the ferrite region, the finishing temperature may be lower than thatof the steel to which Nb or W is added, and the temperature is preferredto be 550°-680° C.

On the other hand, the temperature for coiling the hot rolled steelsheet is preferred to be a range of 400°-600° C., because as saidtemperature is lower, the pickling ability is improved and the picklingcost is reduced and the good surface profile can be ensured, so that thetemperature is preferred to be 400°-600° C.

The reduction in the cold rolling is preferred to be 50-95%.

The recrystallization annealing may be carried out by any process of abox annealing using a bell furnace and a continuous annealing of a rapidheating type, but the continuous annealing is more preferable in view ofthe productivity and the uniformity of the material quality. Theannealing temperature is preferably in a range of 650°-850° C.

Meanwhile, the cooling speed after the soaking, or presence or absenceof the over aging treatment in the case of continuous annealing has nosubstantial influence upon the present invention.

In order to correct the profile of the cold rolled steel sheet afterannealing, a tempering rolling may be additionally conducted under areduction rate of not more than 1.5% through a skinpass.

EXAMPLE

With respect to the compositions shown in Table 2 satisfying therequirements of the invention, molten iron was produced by means of abottom-blown converter and an RH degassing furnace and then continuouslycast or ingot-made and then bloomed to produce a steel slab.

The steel slabs thus obtained were subjected to soaking treatments at atemperature range of 850°-1,080° C. as shown in Table 3. The temperatureof the steel slab prior to the soaking was different and 20°-1,100° C.as shown in this Table.

The thus soaked steel slabs were hot rolled at a hot roll-finishingtemperature of 620°-850° C., and a hot roll-coiling temperature of320°-550° C. to obtain hot rolled sheets each having a thickness of2.8-3.2 mm. Then, the hot rolled sheets were cold rolled to cold rolledsheets each having a thickness of 0.8 mm, and as indicated in Table 3,they were subjected to the recrystallization annealing in a continuousannealing furnace at a uniform temperature of 760°-800° C. All theannealed test sample sheets were treated by a skinpass to obtain thefinal products.

The average properties of the final product in the rolling direction,and in the directions of 45° and 90° to the rolling are shown in Table4.

                                      TABLE 2                                     __________________________________________________________________________    Test Sample                                                                          Chemical Composition (wt %)                                            Steel No.                                                                            C   Si Mn P  S  N  Nb Cr Ti Al B   W                                   __________________________________________________________________________    A      0.0013                                                                            0.02                                                                             0.15                                                                             0.01                                                                             0.008                                                                            0.004                                                                            0.006                                                                            -- -- 0.041                                                                            --  --                                  B      0.0024                                                                            0.02                                                                             0.15                                                                             0.08                                                                             0.006                                                                            0.003                                                                            0.004                                                                            -- -- 0.025                                                                            --  --                                  C      0.0032                                                                            0.02                                                                             0.15                                                                             0.01                                                                             0.008                                                                            0.004                                                                            -- 0.046                                                                            -- 0.035                                                                            --  --                                  D      0.0027                                                                            0.02                                                                             0.13                                                                             0.01                                                                             0.008                                                                            0.002                                                                            -- 0.070                                                                            -- 0.063                                                                            --  --                                  E      0.0032                                                                            0.02                                                                             0.15                                                                             0.01                                                                             0.005                                                                            0.002                                                                            -- -- 0.036                                                                            -- --  --                                  F      0.0012                                                                            0.02                                                                             0.18                                                                             0.01                                                                             0.011                                                                            0.005                                                                            -- -- -- -- 0.0052                                                                            --                                  G      0.0025                                                                            0.90                                                                             0.15                                                                             0.01                                                                             0.009                                                                            0.003                                                                            -- -- -- 0.024                                                                            --  0.004                               H      0.0010                                                                            0.02                                                                             0.15                                                                             0.01                                                                             0.007                                                                            0.001                                                                            -- -- -- 0.056                                                                            --  --                                  I      0.0008                                                                            0.02                                                                             0.15                                                                             0.09                                                                             0.008                                                                            0.003                                                                            -- -- -- 0.028                                                                            0.0035                                                                            --                                  J      0.0027                                                                            0.02                                                                             0.15                                                                             0.01                                                                             0.008                                                                            0.004                                                                            0.002                                                                            -- 0.020                                                                            -- --  --                                  K      0.0046                                                                            0.02                                                                             0.70                                                                             0.07                                                                             0.008                                                                            0.004                                                                            -- 0.052                                                                            0.025                                                                            -- --  --                                  L      0.0023                                                                            0.03                                                                             0.20                                                                             0.05                                                                             0.007                                                                            0.003                                                                            -- -- 0.052                                                                            0.016                                                                            --  --                                  __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                 Steel slab temper-    Hot rolling                                Test Process for                                                                           ature prior to        Finishing                                                                            Coiling                                                                              Annealing conditions         sample                                                                             manufacturing                                                                         being charged into                                                                      Soaking temperature                                                                       temperature                                                                          temperature  Temperature            steel                                                                              steel slab                                                                            soaking pit (°C.)                                                                of steel slab (°C.)                                                                (°C.)                                                                         (°C.)                                                                         Method                                                                              (°C.)           __________________________________________________________________________    A    continuous                                                                             20       1080        780    550    continuous                                                                          800                         casting                                                                  B    continuous                                                                            1100      1100˜1060                                                                           880    500    "     760                         casting                                                                  C    continuous                                                                             22        850        630    420    "     800                         casting                                                                  D    ingot making-                                                                          23       1020        780    550    "     760                         blooming                                                                 E    continuous                                                                            450       1030        660    520    "     800                         casting                                                                  F    continuous                                                                            850        950        650    380    "     770                         casting                                                                  G    continuous                                                                            380       1020        650    400    "     800                         casting                                                                  H    continuous                                                                             25       1000        670    510    "     800                         casting                                                                  I    continuous                                                                             25        950        620    470    "     800                         casting                                                                  J    continuous                                                                            870        860        650    320    "     770                         casting                                                                  K    continuous                                                                            520       1020        850    550    "     800                         casting                                                                  L    continuous                                                                             20       1050        660    530    "     780                         casting                                                                  __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Test                                                                              Yield Tensile         Aging Index                                         sample                                                                            strength                                                                            strength                                                                            Elongation                                                                              AI value                                            steel                                                                             (kg/mm.sup.2)                                                                       (kg/mm.sup.2)                                                                       (%)   -r value                                                                          (kg/mm.sup.2)                                                                        n value*                                     __________________________________________________________________________    A   13    29    54    2.0 1.2    0.31                                         B   21    37    44    1.9 1.6    0.26                                         C   14    28    53    1.8 2.4    0.29                                         D   14    29    52    2.0 1.5    0.28                                         E   15    29    51    2.1 0.4    0.31                                         F   13    28    54    1.7 2.0    0.30                                         G   22    36    43    1.8 0.3    0.25                                         H   14    27    51    1.8 1.0    0.28                                         I   19    36    43    1.8 0.8    0.27                                         J   14    29    53    2.2 1.0    0.29                                         K   25    42    38    1.7 1.5    0.25                                         L   18    32    47    1.8 0.5    0.27                                         __________________________________________________________________________     Note: *n values at 10% and 20% of deformations.                          

As seen from the property values of the materials shown in Table 4, thetensile strengths depend upon the compositions of the test sample steelsand the test sample steels B, G, I, and K show the values of not lessthan 35 kg/mm² and the other samples are not more than 32 kg/mm², butany sample steels are low in the yield strength and high in both theelongation, r value and n value, and show the aging index (AI) of notmore than 3 kg/mm². This indicates that all samples A-L are cold rolledsteel sheets having excellent stretch formability, deep-drawability andaging resistance.

The steel slabs shown in the above Example are ones having a thicknessof about 100-250 mm which are produced by the ingot making-bloomingmethod or a continuous casting method but the invention is obviouslyapplicable to a sheet bar having a thickness of 20-60 mm produceddirectly from the molten steel through a sheet bar caster.

That is, when the sheet bar is subjected to the hot rolling, it ismerely necessary to uniformly heat the bar within a temperature range of800°-1,100° C. or to keep the temperature at said temperature range.Further, the cold rolled steel sheets according to the invention areused effectively as raw materials for manufacturing all sorts of thesurface treating steel sheets such as continuous hot-dip galvanizingsteel sheets by the in-line annealing system.

According to the invention, a cold rolled steel sheet having excellentstretch formability, deep-drawability and aging resistance can bemanufactured only by effecting the soaking treatment at a temperaturerange of 800°-1,100° C. when hot rolling a steel slab in which at leastone of Nb, Cr, Ti, Al, B, and W is added in a total amount of0.002-0.15% to an extremely low carbon steel having a carbon content of0.005% or less without being influenced by the subsequent hot rollingand cold rolling conditions and the annealing conditions.

INDUSTRIAL APPLICABILITY

As mentioned above, the temperature range for the soaking treatmentaccording to the invention is a low temperature range which is contraryto the conventionally common knowledge, and therefore not only a hugeamount of energy consumption can be saved to a large extent, but alsodue to the reduction in the amount of surface oxidation, the yield andthe properties of the surface and interior of the product can be largelyimproved.

We claim:
 1. A process of manufacturing a cold rolled steel sheet havingexcellent press-formability, which comprises soaking a steel slabconsisting of not more than 0.005% by weight of C, not more than 1.20%by weight of Si, 0.05-1.00% by weight of Mn, not more than 0.150% byweight of P, at least one element selected from the group consisting ofNb, Cr, Ti, Al, B, and W in a total amount of 0.002-0.150% by weight,and the balance being Fe and incidental impurities at a temperaturerange of 800°-1,080° C., followed by a hot rolling, a cold rolling, anda recrystallization annealing consecutively, whereby the resulting coldrolled steel sheet has an elongation of over 50% and an r value ≧1.8. 2.A process as claimed in claim 1, wherein the steel slab is soaked at atemperature of 800°-1,080° C., hot rolled at a finishing temperature of550° C.-Ar₃, and a coiling temperature of not higher than 600° C. andsuccessively cold rolled and continuously annealed.
 3. A process ofmanufacturing a cold rolled steel sheet having excellentpress-formability, which comprises soaking a steel slab consisting ofnot more than 0.004% by weight of C, not more than 1.2% by weight of Si,0.05-1.00% by weight of Mn, not more than 0.150% by weight of P,0.005-0.080% by weight of Al, and at least one of Nb and W in a totalamount of 0.002-0.020% by weight, and the balance being Fe andincidental impurities at 800°-1,080° C., followed by a hot rolling at afinishing temperature of 550° C.-Ar₃ l and a coiling temperature of notmore than 600° C., a cold rolling and a continuous annealing, wherebythe resulting cold rolled steel sheet has an elongation of over 50% andan r value ≧1.8.
 4. A process of manufacturing a cold rolled steel sheethaving excellent press-formability which comprises soaking a steel slabconsisting of not more than 0.004% by weight of C, not more than 1.20%by weight of Si, 0.05-1.00% by weight of Mn, not more than 0.150% byweight of P, at least one element selected from the group consisting ofCr, Ti, Al and B in a total amount of 0.002-0.090% by weight, and thebalance being Fe and incidental impurities at 800°-1,050° C., followedby a hot rolling at a finishing temperature of 550°-680° C. and at acoiling temperature of not more than 600° C., a cold rolling and acontinuous annealing, whereby the resulting cold rolled steel sheet hasan elongation of over 50% and an r value ≧1.8.